The present invention relates to light sensors incorporating a charge integrating photodiode as a light transducer.
A light sensor generates an output signal indicating the intensity of light incident upon the light sensor. The light sensor includes a light transducer for converting light into an electrical signal and may also include electronics for signal conditioning, compensation for cross-sensitivities such as temperature, and output signal formatting. Light sensors are used in a wide range of applications including remote sensing, communications, and controls.
One application for light sensors is in automatically dimming vehicle rearview mirrors. Vehicle operators use interior and exterior rearview mirrors to view scenes behind the vehicle without having to face in a rearward direction and to view areas around the vehicle that would otherwise be blocked by vehicle structures. As such, rearview mirrors are an important source of information to the vehicle operator. Bright lights appearing in a scene behind the vehicle, such as from another vehicle approaching from the rear, may create glare in a rearview mirror that can temporarily visually impair or dazzle the operator. This problem is generally worsened during conditions of low ambient light, such as those that occur at night, when the eyes of the vehicle operator have adjusted to the darkness.
Automatically dimming rearview mirrors eliminate the need for the operator to manually switch the mirror. The earliest designs used a single glare sensor facing rearward to detect the level of light striking the mirror. This design proved to be inadequate since the threshold perceived by the operator for dimming the mirror, known as the glare threshold, varied as a function of the ambient light level. An improvement included a second light sensor for detecting the ambient light level. The glare threshold in these systems is based on the amount of ambient light detected. Among the dual sensor designs proposed include those described in U.S. Pat. Nos. 3,601,614 to Platzer; 3,746,430 to Brean et al.; 4,580,875 to Bechtel et al.; 4,793,690 to Gahan et al.; 4,886,960 to Molyneux et al.; 4,917,477 to Bechtel et al.; 5,204,778 to Bechtel; 5,451,822 to Bechtel et al.; and 5,715,093 to Schierbeek et al., each of which is incorporated herein by reference.
A key element in the design of an automatic dimming mirror is the type of light transducer used to implement ambient light and glare detection. A primary characteristic of interest in selecting a light transducer type is the dynamic range. The ratio between the intensity of bright sunlight and moonlight is roughly 1,000,000:1, indicating the wide range that must be sensed by the ambient light sensor. Both the ambient light and the glare light sensors must operate within the ranges of temperature, humidity, shock, and vibration experienced within a vehicle passenger compartment. If a sensor is to be mounted in an outside mirror, even harsher operating conditions can be expected. Sensors and support electronics must also be inexpensive to allow the cost of an automatically dimmed mirror to fall within the range deemed acceptable by an automobile purchaser. Transducers should have good noise immunity or be compatible with noise compensation electronics within the sensor for sensitivity at low light levels. Transducers should further have a spectral response similar to the frequency response of the human eye. As a final desirable characteristic, the sensor must be easily integratable into the types of digital control systems commonly found in automotive applications.
Photodiode light sensors incorporate a silicon-based photodiode and conditioning electronics on a single substrate. The photodiode generates charge at a rate proportional to the amount of incident light. This light-induced charge is collected over an integration period. The resulting potential indicates the level of light to which the sensor is exposed over the integration period. Light sensors with integral charge collection have many advantages. By varying the integration time, the sensor dynamic range is greatly extended. Also, the ability to incorporate additional electronics on the same substrate as the photodiode increases noise immunity and permits the sensor output to be formatted for use by a digital circuit. Component integration additionally reduces the system cost. Silicon light sensors are relatively temperature invariant and can be packaged to provide the necessary protection from humidity, shock, and vibration. One disadvantage of silicon-based light transducers is a frequency response different from that of the human eye. A variety of charge integrating photodiode devices have been described including those in U.S. Pat. Nos. 4,916,307 to Nishibe et al.; 5,214,274 to Yang; 5,243,215 to Enomoto et al.; 5,338,691 to Enomoto et al.; and 5,789,737 to Street, each of which is incorporated herein by reference.
One difficulty with all types of light sensors is the occurrence of operating anomalies at high temperatures. Some devices become extremely non-linear at high temperatures. Some devices may suffer a permanent change in operating characteristics. Devices may even provide completely false readings such as indicating bright light in low light conditions due to excessive thermal noise. Traditionally, the only way to deal with this problem has been to incorporate a temperature sensor and associated electronics into systems that use light sensors.
What is needed is a light sensor with a wide dynamic range that may be incorporated into cost sensitive digital systems such as automatically dimming rearview mirrors. The light sensor should compensate for temperature cross-sensitivity and, preferably, provide an indication of light sensor temperature. A charge integrating light sensor having an externally determined integration period is also desirable.
It is an object of the present invention to provide a charge integrating light sensor with a wide dynamic range.
Another object of the present invention is to provide a packaged light sensor that is economical to produce.
Still another object of the present invention is to provide a charge integrating light sensor that will easily interface to digital electronics.
Yet another object of the present invention is to provide a charge integrating light sensor with an output signal indicating incident light intensity and sensor temperature.
A further object of the present invention is to provide a charge integrating light sensor with an externally determined integration period.
In carrying out the above objects and other objects and features of the present invention, a light sensor is provided. The light sensor includes an exposed photodiode light transducer accumulating charge in proportion to light incident over an integration period. Sensor logic determines the light integration period prior to the beginning of integration. The charge accumulated in the exposed light transducer at the beginning of the light integration period is reset. The charge accumulated by the exposed light transducer over the light integration period is measured and a pulse having a width based on the accumulated charge is determined.
In an embodiment of the present invention, the light sensor includes a comparator with one input connected to the exposed light transducer and the other input connected to a switched capacitor circuit. The switched capacitor circuit charges a capacitor to a fixed voltage when the switch is closed and discharges the capacitor at a constant rate when the switch is open. The sensor logic closes the switch during the light integration period and opens the switch after the light integration period, thereby creating the pulse at the comparator output. In a refinement, the light sensor further includes a second comparator with one input connected to a fixed voltage and the other input connected to the switched capacitor circuit. The second comparator output inhibits output of the determined pulse if the ramp voltage is less than the fixed voltage.
In another embodiment of the present invention, the light sensor includes a photodiode light transducer shielded from light. The shielded light transducer accumulates charge in proportion to noise over the integration period. The sensor logic resets charge accumulated in the shielded light transducer at the beginning of the light integration period. Charge accumulated by the shielded light transducer over the light integration period is measured and an output pulse having a width based on the difference between the exposed light transducer charge and the shielded light transducer charge is determined.
In still another embodiment of the present invention, the light sensor has an input for receiving an integration signal. Since the noise is dependent on the light sensor temperature, the output pulse can be used to indicate sensor temperature. The output pulse is sent following the end of the received integration signal after a length of time based on the noise level.
In yet other embodiments of the present invention, the light integration period may be determined from the asserted portion of a control signal received by the sensor logic or may be determined within the sensor control by cycling through a sequence of predetermined time periods.
In a further embodiment of the present invention, the light sensor includes at least one additional exposed photodiode light transducer. Each additional exposed light transducer accumulates charge in proportion to light incident over an integration period at a rate different from the rate of any other exposed light transducer. The sensor logic outputs a pulse having a width based on the accumulated charge for each of the additional exposed light transducers. In one refinement, each exposed light transducer has a different collector area. In another refinement, each exposed light transducer has an aperture with a different light admitting area.
A light sensor package is also provided. The package includes an enclosure having a window for receiving light. The enclosure admits a power pin, a ground pin, and an output pin. Within the enclosure, an exposed photodiode light transducer accumulates charge in proportion to light received through the window incident over the integration period. A light-to-voltage circuit outputs a light voltage signal based on charge accumulated by the exposed light transducer. A voltage-to-pulse circuit outputs a pulse on the output pin. The width of the pulse is based on the light voltage signal.
A light sensor with a photodiode overlaying a substrate is also provided. The photodiode accumulates charge generated by light incident on the photodiode in a photodiode well formed in a region of the substrate underlying the photodiode. The photodiode has an intrinsic photodiode capacitance. A floating diffusion having an intrinsic floating diffusion capacitance is also formed in the substrate. The floating diffusion has a diffusion well formed in a region of the substrate underlying the floating diffusion when the charge is reset. The floating diffusion eliminates charge in the diffusion well when the charge is reset. The floating diffusion charge determines an output potential. A transmission gate having an intrinsic transmission gate capacitance is placed between the photodiode and the floating diffusion. The transmission gate forms a transmission well in a region of the substrate between the region of the substrate underlying the photodiode and the region of the substrate underlying the floating diffusion. The transmission well has a depth shallower than the photodiode well and the diffusion well. When the charge is reset, charge in the photodiode well above the depth of the transmission well flows through the transmission well, through the floating diffusion, and is eliminated. During a light integration period, charge produced by light incident on the photodiode flows through the transmission well and into the diffusion well, producing output voltage inversely proportional to the floating diffusion capacitance. Once the diffusion well is filled to the depth of the transmission well, charge produced by light incident on the photodiode fills the photodiode well, the transmission well, and the diffusion well, producing output voltage inversely proportional to the sum of the floating diffusion capacitance, the photodiode capacitance, and the transmission gate capacitance. This dual capacitance provides a first sensitivity during charge accumulation in the diffusion well only and a second sensitivity during charge accumulation in the diffusion well, the transmission well, and the photodiode well. The first sensitivity is greater than the second sensitivity.
In an embodiment, the light sensor includes an anti-bloom gate between the photodiode and a source voltage diffusion. The anti-bloom gate defines an anti-blooming well formed in a region of the substrate between the region of the substrate underlying the photodiode and the source voltage diffusion. The anti-blooming well has a depth shallower than the transmission well.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.